It is frequently necessary to determine the concentration of various blood analytes when maintaining or treating mammals, including humans. An important example is the diabetic, whose glucose must constantly be monitored. Presently, diabetic blood is sampled invasively, typically through a finger prick or by drawing a blood sample. Other analytes of potential interest include lipids, cholesterol, serum proteins and electrolytes.
Much work has been done to monitor glucose by various non-invasive methods. These methods include infrared spectrophotometry. See Barnes et al., U.S. Pat. No. 5,070,874 (assigned to Biocontrol Technology, Inc.), Rosenthal et al., U.S. Pat. No. 5,028,787 (assigned to Futrex, Inc.), Dahne et al., European Patent Publication No. 0,160,768 (Batelle Memorial Institute), and Robinson et al., U.S. Pat. No. 4,975,581 (assigned to the University of New Mexico). Other methods include alternative optical means such as that proposed in the article "Noninvasive Glucose Monitoring of the Aqueous Humor of the Eye" by Rabinovitch et al., Diabetes Care, Vol. 5 (No. 3): pp. 254-258 (May-June 1982). Other methods have been devised, including sampling of interstitial fluid in the mouth (unpublished communication from Mary Anne MacGillivray and Richard Battelle of Healthcraft International, Pasadena, Calif., 1988). Although non-invasive measurements can be made, all known prior art methods fail to distinguish whether an analyte, e.g. glucose, is in the subject's bloodstream or in surrounding tissue. This can result in incorrect and inappropriate determinations because of chemical interactions of the same or a spectrophometrically similar analyte located in the tissue where transport and utilization is distinctly different from the analyte moving through veins and arteries.
In addition, all prior methods suffer from low signal to noise of the spectrophotometric signals due to limitations of the light source and the limited resolution of spectral means such as gratings, prisms, filters, Hadamard or Fourier Transform (interferometric) spectrometers. These limitations result in poor analyte resolution or lengthy sample times, unnecessarily confining or restricting the movements of the subject or patient being tested.
In the known, non-invasive measurement prior art, no effort is made to distinguish analyte in the blood from that in tissue or interstitial fluids. Additionally, previous workers such as Robinson, Rosenthal and others limit spectroscopic non-invasive measurements to the near infrared region of the electromagnetic spectrum ignoring the visible and the medium and long infrared regions, limiting performance, accuracy, and range of blood analytes which can be detected and resolved.
Additionally, many medical conditions cause a physiological change in certain tissues which are otherwise suitable for non-invasive monitoring. For example, diabetics frequently suffer substantial reductions in peripheral blood flow to peripheral tissue such as the ear or finger.
Current methods and devices fail to provide adequate information about blood flows, blood volumes, and tissue temperatures at and across the tissue field. This information may be important to the determination of blood analytes and/or the proper medical evaluation of the blood analyte level and/or the medical condition, and temperature dependant behavior of the analyte and tissue. Current methods and devices also fail to provide information about the blood analytes located specifically in the tissue and interstitial fluid.